The present invention relates to the field of gas turbine engines and is concerned more specifically with the arrangement of a semiconductor-type igniter plug mounted in a combustion chamber.
In a gas turbine engine, the combustion chamber receives air from the compressor, some of which air is mixed with the fuel which is burnt in the primary combustion zone. Ignition is brought about by one or two igniter plugs positioned downstream of the carburetion system. Another part of the air bypasses the primary combustion zone and mixes with the combustion gases. All of the hot gases are directed toward the turbine. The combustion chambers are designed to meet a certain number of essential specifications such as: in-flight reignition, the shape of the temperature profile, the emissions of pollutant gases and both thermal and mechanical integrity of the various components thereof.
In particular, the ignition system must be capable of in-flight reignition if the combustion chamber is accidently extinguished while at the same time being able to tolerate and withstand the thermal stresses applied to it. These two conditions dictate arrangements that are difficult to reconcile. Specifically, the injection system produces a sheet of atomized fuel at a certain angle. If this angle is very tight, the igniter plug is outside of the cone formed by the fuel, which is favorable in terms of thermal integrity but reduces the ease of ignition within the chamber. Conversely, an injection system in which the sheet of fuel forms a very wide cone angle considerably heats up the chamber zone surrounding the igniter plug because of fuel impinging on the walls and the igniter plug. This has an adverse effect on the thermal integrity of these components.
The present invention is concerned with ignition systems in which the igniter plug is mounted on the chamber via a component that acts as an adapter itself attached to the casing of the chamber. The igniter plug extends from the casing radially toward the inside of the chamber and its end lies flux with the wall of the chamber through an opening formed therein and that forms a duct. A minimal lateral clearance is left around the igniter plug in order to allow relative movement between the chamber and the casing as a result of temperature variations during the various phases of flight without the igniter plug, which is secured to the casing, butting or pressing against the edges of the opening in the wall of the chamber. The opening in the wall of the chamber forms a duct into which the igniter plug is slipped and a floating sleeve surrounding the igniter plug provides sealing between the chamber and the space between the chamber and the casing. An example of this way of mounting an igniter plug in a combustion chamber using an adapter is disclosed in patent application EP 1.443.190.
It may be noted that there are two types of igniter plug used in gas turbine engine combustion chambers:
Igniter plugs of the “high energy/high voltage” type, the operating voltage of which is of the order of 20 kV, and igniter plugs of the “high energy/low voltage” type, the operating voltage of which is of the order of 2 to 3 kV. Igniter plugs of the “high energy/low voltage” type have a semiconductor material between their electrodes, such that if sufficient voltage is applied between the electrodes, a spark is created. The life of the igniter plugs is limited; in the case of “high energy/high voltage” igniter plugs, this is limited by electrode wear and in the case of “high energy/low voltage” igniter plugs, life is limited by semiconductor wear, the semiconductor becoming worn away more rapidly than the electrodes.
The advantages of “high energy/low voltage” igniter plugs are, on the one hand, associated with the fact that their operation is not very dependent on combustion chamber conditions and, on the other hand, associated with the more compact ignition chain that need be installed in order to operate them.
After repeated ignition sequences, the igniter plug semiconductor experiences significant thermal stresses. The impingement of fuel on the semiconductor, together with the electric arc cause damage to this semiconductor, altering its shape.